1. Field of the Invention
This invention generally relates to methods for the pharmacological treatment of breathing disorders and, more specifically, to the administration of agents or compositions having serotonin-related receptor activity for the alleviation of sleep apnea (central and obstructive) and other sleep-related breathing disorders.
2. Related Technology
Over the past several years much effort has been devoted to the study of a discrete group of breathing disorders that occur primarily during sleep with consequences that may persist throughout the waking hours in the form of sleepiness, thereby manifesting itself into substantial economic loss (e.g., thousands of lost man-hours) or employment safety factors (e.g., employee non-attentiveness during operation of heavy-machinery). Sleep-related breathing disorders are characterized by repetitive reduction in breathing (hypopnea), periodic cessation of breathing (apnea), or a continuous or sustained reduction in ventilation.
In general sleep apnea is defined as an intermittent cessation of airflow at the nose and mouth during sleep. By convention, apneas of at least 10 seconds in duration have been considered important, but in most individuals the apneas are 20-30 seconds in duration and may be as long as 2-3 minutes. While there is some uncertainty as to the minimum number of apneas that should be considered clinically important, by the time most individuals come to attention of the medical community they have at least 10 to 15 events per hour of sleep.
Sleep apneas have been classified into three types: central, obstructive, and mixed. In central sleep apnea the neural drive to all respiratory muscles is transiently abolished. In obstructive sleep apneas, airflow ceases despite continuing respiratory drive because of occlusion of the oropharyngeal airway. Mixed apneas, which consist of a central apnea followed by an obstructive component, are a variant of obstructive sleep apnea. The most common type of apnea is obstructive sleep apnea.
Obstructive sleep apnea syndrome (OSAS) has been identified in as many as 24% of working adult men and 9% of similar women, with peak prevalence in the sixth decade. Habitual heavy snoring, which is an almost invariant feature of OSAS, has been described in up to 24% of middle aged men, and 14% of similarly aged women, with even greater prevalence in older subjects.
Obstructive sleep apnea syndrome's definitive event is the occlusion of the upper airway, frequently at the level of the oropharynx. The resultant apnea generally leads to a progressive-type asphyxia until the individual is briefly aroused from the sleeping state, thereby restoring airway patency and thus restoring airflow.
An important factor that leads to the collapse of the upper airway in OSAS is the generation of a critical subatmospheric pressure during the act of inspiration that exceeds the ability of the airway dilator and abductor muscles to maintain airway stability. Sleep plays a crucial role by reducing the activity of the muscles of the upper airways including the dilator and abductor muscles.
In most individuals with OSAS the patency of the airway is also compromised structurally and is therefore predisposed to occlusion. In a minority of individuals the structural compromise is usually due to obvious anatomic abnormalities, i.e, adenotonsillar hypertrophy, retrognathia, or macroglossia. However, in the majority of individuals predisposed to OSAS, the structural abnormality is simply a subtle reduction in airway size, i.e., "pharyngeal crowding." Obesity also frequently contributes to the reduction in size seen in the upper airways. The act of snoring, which is actually a high-frequency vibration of the palatal and pharyngeal soft tissues that results from the decrease in the size of the upper airway lumen, usually aggravates the narrowing via the production of edema in the soft tissues.
The recurrent episodes of nocturnal asphyxia and of arousal from sleep that characterize OSAS lead to a series of secondary physiologic events, which in turn give rise to the clinical complications of the syndrome. The most common manifestations are neuropsychiatric and behavioral disturbances that are thought to arise from the fragmentation of sleep and loss of slow-wave sleep induced by the recurrent arousal responses. Nocturnal cerebral hypoxia also may play an important role. The most pervasive manifestation is excessive daytime sleepiness. OSAS is now recognized as a leading cause of daytime sleepiness and has been implicated as an important risk factor for such problems as motor vehicle accidents. Other related symptoms include intellectual impairment, memory loss, personality disturbances, and impotence.
The other major manifestations are cardiorespiratory in nature and are thought to arise from the recurrent episodes of nocturnal asphyxia. Most individuals demonstrate a cyclical slowing of the heart during the apneas to 30 to 50 beats per minute, followed by tachycardia of 90 to 120 beats per minute during the ventilatory phase. A small number of individuals develop severe bradycardia with asystoles of 8 to 12 seconds in duration or dangerous tachyarrhythmias, including unsustained ventricular tachycardia. OSAS also aggravates left ventricular failure in patients with underlying heart disease. This complication is most likely due to the combined effects of increased left ventricular afterload during each obstructive event, secondary to increased negative intrathoracic pressure, recurrent nocturnal hypoxemia, and chronically elevated sympathoadrenal activity.
Central sleep apnea is less prevalent as a syndrome than OSAS, but can be identified in a wide spectrum of patients with medical, neurological, and/or neuromuscular disorders associated with diurnal alveolar hypoventilation or periodic breathing. The definitive event in central sleep apnea is transient abolition of central drive to the ventilatory muscles. The resulting apnea leads to a primary sequence of events similar to those of OSAS. Several underlying mechanisms can result in cessation of respiratory drive during sleep. First are defects in the metabolic respiratory control system and respiratory neuromuscular apparatus. Other central sleep apnea disorders arise from transient instabilities in an otherwise intact respiratory control system.
Many healthy individuals demonstrate a small number of central apneas during sleep, particularly at sleep onset and in REM sleep. These apneas are not associated with any physiological or clinical disturbance. In individuals with clinically significant central sleep apnea, the primary sequence of events that characterize the disorder leads to prominent physiological and clinical consequences. In those individuals with central sleep apnea alveolar hypoventilation syndrome, daytime hypercapnia and hypoxemia are usually evident and the clinical picture is dominated by a history of recurrent respiratory failure, polycythemia, pulmonary hypertension, and right-sided heart failure. Complaints of sleeping poorly, morning headache, and daytime fatigue and sleepiness are also prominent. In contrast, in individuals whose central sleep apnea results from an instability in respiratory drive, the clinical picture is dominated by features related to sleep disturbance, including recurrent nocturnal awakenings, morning fatigue, and daytime sleepiness.
Currently, the most common and most effective treatment, for adults with sleep apnea and other sleep-related breathing disorders are mechanical forms of therapy that deliver positive airway pressure (PAP). Under PAP treatment, an individual wears a tight-fitting plastic mask over the nose when sleeping. The mask is attached to a compressor, which forces air into the nose creating a positive pressure within the patient's airways. The principle of the method is that pressurizing the airways provides a mechanical "splinting" action, which prevents airway collapse and therefore, obstructive sleep apnea. Although an effective therapeutic response is observed in most patients who undergo PAP treatment, many patients cannot tolerate the apparatus or pressure and refuse treatment. Moreover, recent covert monitoring studies clearly demonstrate that long-term compliance with PAP treatment is very poor.
A variety of upper airway and craniofacial surgical procedures have been attempted for treatment of OSAS. Adenotonsillectomy appears to be an effective cure for OSAS in many children, but upper airway surgery is rarely curative in adult patients with OSAS. Surgical "success" is generally taken to be a 50% reduction in apnea incidence and there are no useful screening methods to identify the individuals that would benefit from the surgery versus those who would not derive a benefit.
Pharmacological treatments of several types have been attempted in patients with sleep apnea but, thus far, none have proven to be generally useful. A recent systematic review of these attempts is provided by Hudgel [J. Lab. Clin. Med., 126:13-18 (1995)]. A number of compounds have been tested because of their expected respiratory stimulant properties. These include (1) acetazolamide, a carbonic anhydrase inhibitor that produced variable improvement in individuals with primary central apneas but caused an increase in obstructive apneas, (2) medroxyprogesterone, a progestin that has demonstrated no consistent benefit in OSAS, and (3) theophylline, a compound usually used for the treatment of asthma, which may benefit patients with central apnea but appears to be of no use in adult patients with obstructive apnea.
Other attempted pharmacological treatment includes the administration of adenosine, adenosine analogs and adenosine reuptake inhibitors (U.S. Pat. No. 5,075,290). Specifically, adenosine, which is a ubiquitous compound within the body and which levels are elevated in individuals with OSAS, has been shown to stimulate respiration and is somewhat effective in reducing apnea in an animal model of sleep apnea.
Other possible pharmacological treatment options for OSAS include agents that stimulate the brain activity or are opioid antagonists. Specifically, since increased cerebral spinal fluid opioid activity has been identified in OSAS, it is a logical conclusion that central stimulants or opioid antagonists would be a helpful treatment of OSAS. In reality, doxapram, which stimulates the central nervous system and carotid body chemoreceptors, was found to decrease the length of apneas but did not alter the average arterial oxygen saturation in individuals with obstructive sleep apnea. The opioid antagonist naloxone, which is known to stimulate ventilation was only slightly helpful in individuals with obstructive sleep apnea.
Because OSAS is strongly correlated with the occurrence of hypertension, agents such as angiotensin-converting enzyme (ACE) inhibitors may be of benefit in treating OSAS individuals with hypertension but this does not appear to be a viable treatment for OSAS itself.
Finally, several agents that act on neurotransmitters and neurotransmitter systems involved in respiration have been tested in individuals with OSAS. Most of these compounds have been developed as anti-depressant medications that work by increasing the activity of monoamine neurotransmitters including norepinephrine, dopamine, and serotonin. Protriptyline, a tricyclic anti-depressant, has been tested in several small trials with variable results and frequent and significant side effects. As serotonin may promote sleep and stimulate respiration, tryptophan, a serotonin precursor and selective serotonin reuptake inhibitors have been tested in individuals with OSAS. While a patent has been issued for the use of the serotonin reuptake inhibitor, fluoxetine (U.S. Pat. No. 5,356,934), initial evidence suggests that these compounds may yield measurable benefits in only approximately 50% of individuals with OSAS. Therefore in view of the fact that the only viable treatment for individuals suffering from sleep-related breathing disorders is a mechanical form of therapy (PAP) for which patient compliance is low, and that hopes for pharmacological treatments have yet to come to fruition, there remains a need for simple pharmacologically-based treatments that would offer benefits to a broad base of individuals suffering from a range of sleep-related breathing disorders. There also remains a need for a viable treatment of sleep-related breathing disorders that would lend itself to a high rate of patient compliance.